1. Field of the Invention
The present invention relates to a magnetic recording medium and production thereof. More particularly, it relates to a magnetic recording medium with improved magnetic properties comprising a substrate and, as a recording layer, a ferromagnetic metal thin layer containing oxygen atoms and a method for the production of the magnetic recording medium.
2. Discussion of Prior Art
Generally, a magnetic recording medium comprising a ferromagnetic metal thin layer as a recording layer is produced by vacuum deposition of ferromagnetic metal or an alloy thereof on a substrate such as a plastic film, while it turns around a peripheral surface of a cylindrical member installed in a vacuum deposition apparatus. To produce the magnetic recording medium with improved magnetic properties, the vaporized ferromagnetic metal is obliquely impinged on the substrate and simultaneously oxydizing gas is jetted into a stream of the vaporized metal near a position at which the vaporized metal is impinged on the substrate at the maximum impinging angle (cf. Japanese Patent Kokai Publication (unexamined) Nos. 41442/1983 and 83328/1983) or at the minimum impinging angle (cf. Japanese Patent Kokai Publication (unexamined) Nos. 41443/1983 and 83327/1983).
In the former method, however, the magnetic properties are not satisfactorily improved. Particularly, improvement of coercive force of the recording medium greatly depends on the deposition rate. As the deposition rate increases, the concentration of oxygen atoms contained in the ferromagnetic metal thin layer increases in the interfacial layer adjacent to the substrate and decreases at the surface of the thin layer. Therefore, the effect of the contained oxygen atom is not effectively exerted and it is difficult to produce a magnetic recording medium with improved magnetic properties at such a high deposition rate as is suitable for mass production.
Japanese Patent Kokai Publication (unexamined) No. 83327/1983 describes that the latter method provides a magnetic recording medium in which the concentration of oxygen atom contained in the ferromagnetic metal thin layer gradually increases from the interfacial layer to the surface. According to study by the present inventors, in the magnetic recording medium produced by this method the concentration of oxygen atom contained in the ferromagnetic metal thin layer decreases from the interfacial layer to the surface. Since the degree of decrease of the oxygen atom concentration is smaller than the former method, although the effect of the oxygen atom content is not as seriously effected as in the former method, still improvement of the magnetic properties is not satisfactory. Particularly, it is also difficult to produce the magnetic recording medium with the improved magnetic properties at such a high deposition rate as is suitable for mass production.